Read this book to learn the answers to questions like:
Why do so many problem-solving sites suggest that I repair permissions?
Why can't I always access my own files when I boot from an external drive?
What should I do if someone tells me to "set the permissions to -rw-r--r--"?
What are promiscuous permissions, and should I be informing the vice squad?
What are access control lists (ACLs), and what's new about them in Snow Leopard?
What are the default permissions for copied files?
With the NetInfo database gone, how do I edit account settings such as numeric UID?
How do I work with account groups from System Preferences?